262 research outputs found
Isometric endomorphisms of free groups
An arbitrary homomorphism between groups is nonincreasing for stable
commutator length, and there are infinitely many (injective) homomorphisms
between free groups which strictly decrease the stable commutator length of
some elements. However, we show in this paper that a random homomorphism
between free groups is almost surely an isometry for stable commutator length
for every element; in particular, the unit ball in the scl norm of a free group
admits an enormous number of exotic isometries.
Using similar methods, we show that a random fatgraph in a free group is
extremal (i.e. is an absolute minimizer for relative Gromov norm) for its
boundary; this implies, for instance, that a random element of a free group
with commutator length at most n has commutator length exactly n and stable
commutator length exactly n-1/2. Our methods also let us construct explicit
(and computable) quasimorphisms which certify these facts.Comment: 26 pages, 6 figures; minor typographical edits for final published
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Ziggurats and Rotation Numbers
We establish the existence of new rigidity and rationality phenomena in the theory of nonabelian group actions on the circle and introduce tools to translate questions about the existence of actions with prescribed dynamics into finite combinatorics. A special case of our theory gives a very short new proof of Naimi's theorem (i.e., the conjecture of Jankins-Neumann) which was the last step in the classification of taut foliations of Seifert fibered spaces
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